1. Field of the Invention
The present invention relates to a fuel cell assembly operating method and a fuel cell system adapted for power generation in a subfreezing environment.
2. Description of the Related Art
Referring to FIG. 3, a solid polymer type fuel cell assembly 100 has a solid polymer electrolyte membrane 101, a hydrogen electrode 102 and an oxygen electrode 103 that have catalytic action, which are provided on both sides of the membrane 101, and separators 104 and 105 that constitute supply passages of hydrogen and oxygen (contained in air), which are reactive gases, between the electrodes 102 and 103.
A hydrogen gas H2 supplied to a supply passage 106 formed by the separator 104 emits electrons e− at the hydrogen electrode 102, turning into hydrogen ions H+. The hydrogen ions H+ conduct in the solid polymer electrolyte membrane 101. Meanwhile, in the oxygen electrode 103, the reaction represented by expression (1) given below takes place from an oxygen gas O2 in the air supplied to a supply passage 107 formed by the separator 105 and an electron e− and a hydrogen ion H+ supplied from the oxygen electrode 103, thereby generating water (H2O).½O2+2H++2e−→H2O  (1)
When starting up the fuel cell assembly 100 in a subfreezing environment, if water generated according to the above expression (1) during previous power generation still remains in the fuel cell assembly 100, the remaining water freezes on the oxygen electrode 103, inconveniently leading to deteriorated conductivity of the hydrogen ions H+ in the polymer electrolyte membrane 101. This results in deteriorated power generating performance of the fuel cell assembly 100.
As a method for thawing ice in a fuel cell assembly by increasing the temperature of a fuel cell stack when starting up a fuel cell assembly in a subfreezing environment to solve the aforesaid problem, there has been proposed, for example, a method for heating the air supplied to a fuel cell stack by a heater to raise the temperature of the fuel cell stack (refer to, e.g., Japanese Unexamined Patent Application Publication No. 2002-93445).
However, providing the heater for heating air, as mentioned above, inconveniently complicates the construction of a fuel cell system and also increases the number of components.